thunderbird opens in "set up your existing email address" no inbox or anything else

This issue is rather typical for a corrupted prefs.js file, which is one of the most important files in the Thunderbird profile. The prefs.js file can get damaged because of a Thunderbird crash, a PC crash or power failure. When this happens Thunderbird renames the damaged prefs.js file in prefs-1.js (prefs-x+1.js) or invalidprefs.js and creates a new pristine prefs.js file of a 6 to 7 kB size which doesn't contain any account settings.

When you get the start page prompting to "set up your existing email address" go to Help > Troubleshooting Information > Profile Folder > Open Folder..., then quit Thunderbird . In the profile folder search for a prefs.js file of 6 - 7 kB size with the actual timestamp and a much bigger prefs-x.js with an actual timestamp.

If a damaged prefs.js file is the culprit the easiest solution would be to replace it with the prefs.js file from a recent profile backup.

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